Multicasting is a service that permits sources to send a single copy of the same data to an address that causes the data to be delivered to multiple recipients. Under multicasting only one copy of a message passes over any link in a network and copies of the message are made only where paths diverge. From the network perspective, multicast dramatically reduces overall bandwidth consumption, since the data is replicated in the network at appropriate points rather than in the end-systems. Further a server, which is sending multicast message needs to manage only one session.
Local area networks have supported multicasting for many years. For networks, where nodes share a common communication medium multicasting is easy to support. A specially addressed packet can be read off the communication medium by multiple hosts.
Extending multicasting capabilities to internetworks however led to the introduction of a router at the edge of a network in order to figure out dynamically how to forward the arriving data packets. The way of forwarding is for example derived from the address included in the header of the data packet and from the routing table, which is administrated in the router. There are few possibilities of performing the multicast addressing, for example to use the address indicating the multicast group.
In case the multicast is used in Internet Protocol IP network then it is called IP multicast. Within the IP multicast the membership of a multicast session group is dynamic, which means that the hosts may join and leave groups at any time. In order to allow hosts on networks to indicate whether they wish to join or leave a particular multicast group there is a protocol called the Internet Group Message Protocol IGMP. Thus this protocol lets the system know which hosts currently belong to which multicast group. This information is required by the multicast routers to know which multicast data packet is to be forwarded onto which interface.
The IGMP is a part of the IP layer and the IGMP messages are transmitted in IP data packets. The version 1 of IGMP is described in RFC 1112 “Host extensions for IP multicasting” S. E. Deering, Aug. 1, 1989. RFC 2236 “Internet Group Management Protocol, Version 2” W. Fenner, November 1997 describes the version 2. The IGMP has been developed for IP version 4. In Internet Protocol IP version 6 there is a similar protocol called Multicast Listener Discovery MLD, which is used for the same purpose as the IGMP. The description of the first version of MLD can be found in RFC 2710 “Multicast Listener Discovery (MLD) for IPv6” S. Deering, W. Fenner, B. Haberman, October 1999. However the messages used in MLDcorrespond to the IGMP messages. In the following the IGMP will be used as an example. Although this should not be restricted to the IGMP, the functionality of the invention is also given by usage of MLD.
In principle the IGMP uses two basic messages to fulfil its tasks, the membership report and the membership query message and the following rules are applied. The different versions of IGMP contain also additional messages.
A multicast router sends a membership query at regular intervals to see if any host still belongs to any group. The router must send one query out each interface. The group address in the query is 0 since the router expects one response from a host for every group that contains one or more members on each host. It is also possible to send a membership query for one particular group rather than for all groups. A host responds to an IGMP query by sending one IGMP report for each group that still contains at least one user. A host joins a group also by sending the membership report.
Using the information received by applying the report and the query messages a table with its interfaces having at least one host in a multicast group is established. The router forwards the received multicast data out the interface, which has at least one member.
With the IP multicast the receivers do not need to know who or where the senders are to receive traffic from them and the senders do not need to know who the receivers are. Neither senders nor receivers need to care about the network topology as the network optimises delivery. The distribution of the information via the IP multicast is performed on the base of a hierarchical connection of the hosts, like for example a multicast delivery tree. Several algorithms have been proposed for building multicast distribution trees, like for example spanning trees, shared-trees, source-based trees, core-based trees. The descriptions of the corresponding algorithms can be found in “IP telephony: Packet-based multimedia communications systems” O. Hersent, D. Gurle, D. Petit, Addison-Wesley, Harlow, 2000. After the establishment of the multicast delivery tree, the distribution of the information is done by the IP multicast routing protocols. The detailed description of the corresponding IP multicast routing protocols can also be found in the above-mentioned document.
An advantage of the IP multicast is the support of the heterogeneous receivers. With IP multicast it is possible to send different media to different multicast groups and the receivers decide which media to receive in dependence on the own capability and/or preferences. Similarly, if a sender layers its video or audio stream, different receivers can choose to receive different amounts of traffic and hence different qualities. To do this the sender must code the video as a base layer it means with the lowest quality that might be acceptable and a number of enhancement layers, each of which adds more quality at the expense of more bandwidth. With video, these additional layers might increase the frame rate or increase the spatial resolution of the images or both. Each layer is sent to a different multicast group and receivers can individually decide how many layers to subscribe to and consequently to receive.
Multicasting in the internetworking between a fixed network and a mobile network like, General Packet Radio System GPRS or Universal Mobile Communication System UMTS causes some further problems. The impacts on the problems have for example the mobility of the end users and the low transmission bandwidth of the mobile network on the air interface. Further the communication in a mobile communication networks like for example in UMTS is a unicast communication. The unicast communication is also called point-to-point communication. The point-to-point communication means sending of a message from a single sender to a single receiver. In such kind of network, in particular in the core network it is not foreseen to perform a multicast communication. The group communication is implemented by means of a point-to-point communication having a sender transmitting separately packets to each member of the group. For a group with n members, n packets are required on the whole way between the sender and the receivers, instead of one packet when multicast transmission is used.
In order to explain the problem occurring in a point-to-point oriented telecommunication system in the following an overview of the architecture of the General Packet Radio System GPRS network is given.
The GPRS is the packet-switched enhancement of the Global System for Mobile Communication GSM, which is a circuit switched network. It means that the user can be permanently online connected but has to pay only for the real data transfer. In order to fulfil the new requirements some changes are introduced into the GSM inter alia new logical nodes, the Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node (GGSN) are introduced. The main functions of the GGSN involve interaction with external IP packet networks providing for example connections to Internet Service Providers ISPs. From the external IP network's point of view, the GGSN acts as a router for the IP addresses of all subscribers served by the GPRS networks. The GGSN also exchanges routing information with the external network. The SGSN serves all GPRS subscribers that are physically located within the geographical SGSN service area. It forwards incoming and outgoing IP packets addressed to or from a mobile station. Additional to the new logical nodes also new interfaces between the nodes are to be defined. For the invention in particular the Gn, Gi, Gp interfaces are relevant. The Gp interface is defined between GGSN nodes belonging to different operators. The Gn interface defines the IP based backbone between the SGSN and GGSN. The Gi is the interface between GGSN and a further network, like Internet. The restriction of GPRS is that GGSN and SGSN are to be connected in a way that IP is run on top of the technology chosen, meaning that SGSN and GGSN communicate via IP addresses. A detailed description of the architecture is to be found in 3GPP TS 03.60 V7.5.0 (2001-01) 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, Digital cellular Telecommunications System (Phase 2+), General Packet Radio Service (GPRS), Service Description, Stage 2 (Release 1998).
The similar nodes and interfaces are also used in the next generation of the wireless networks, in UMTS as described in 3GPP TS 23.060 V3.6.0 (2001-01) 3rd Generation Partnership Project; Technical Specification Group Services and System Aspects, General Packet Radio Service (GPRS), Service Description, Stage 2 (Release 1999). In order to distinguish between the functionality of these nodes in UMTS extended names are often used, 3G-SGSN and 3G-GGSN. In the following description it will not be distinguished between the GPRS and UMTS nodes.
In the following an overview of an UMTS network, as specified in the 3GPP specifications, UMTS Standard 23.060 as mentioned above, in respect to FIG. 1 is given.
The FIG. 1 shows the core network as packet switching domain, indicated as Packet. The core network is connected to a radio network, depicted as Radio NW. On top of the packet switching domain of the core network, there is the IP Multimedia Subsystem (IMS), IP Multimedia for conversational multimedia services. Each of the subsystem includes the corresponding nodes. Relevant for the present invention are the nodes of the core network, the SGSN and the GGSN node with the involved interface Gn, Gi, which will be further described in more detail. The also relevant Gp interface is not depicted in FIG. 1. IMS as an example uses the packet switching domain to provide bearers for the conversational multimedia services. Streaming multimedia services are also possible without IMS by using for example a streaming server in the Internet on top of a corresponding packet switched bearer.
With the introduction of streaming and conversational multimedia services, many new point-to-multipoint services will evolve. These will have high demands on the network infrastructure and consume considerable amounts of bandwidth. Some examples of such services are video-conferencing, whiteboarding, real-time multi-user games, multimedia messaging, virtual worlds.
According to FIG. 1 an external IP network, like the Internet is depicted as Multimedia/IP Network, the mobile station as TE and the core network as Packet. Currently the IP multicast messages in the UMTS are sent from a router settled in an external IP network transparently to the mobile station via a unicast connection. As already mentioned the multicast is performed on the IP layer and from the view of the mobile station TE the router in the Internet is the first node in which the IP connection terminates and therefore the first node applicable for the multicast. It means the IP layer in the GGSN, which allows the communication to the external networks, is currently not seen as capable for performing multicast. The router sends multicast messages within the core part of the mobile network without distinguishing between a multicast message and a unicast messages. The separation of the multicast flows is already done in the router in the Internet and the same data packets are sent over the wireless network multiple times depending on the number of recipients.
It means the existing UMTS technology does not foresee the utilisation of efficient multicasting on the part of the network denoted by the Gn interface in the FIG. 1. Any service that is simultaneously provided to multiple clients is replicated at the edge of the wireless network and multiple unicast connections are used towards the clients. Especially with the evolution of resources high-demanding streaming or conversational multimedia services this implies that the resources in the network are used very inefficiently.
Further the existing nodes are not prepared for performing a multicasting.
In general introducing and performing of multicast in a network, that is basically point-to-point oriented causes problems, than in such kind of networks a unicast channel is established for performing a communication between two nodes. It means the problem does not only occur in a wireless network like UMTS.
Further examples of protocols being multicast capable, are SIP (session Initiation Protocol) or RTSP (Real-time Streaming Protocol). The SIP protocol is described in Multiparty Multimedia Session Control (MMUSIC) WG in IETF, and the RTSP is covered by RFC 2326 Real Time Streaming Protocol (RTSP) H. Schulzrinne, A. Rao, R. Lanphier, April 1998. These protocols belong also to point-to-point oriented protocols and the following invention applies also for them.